1. Field of the Invention
The present invention relates to a process for preparing cyclic alcohols and ketones having from 7-16 carbons by oxidizing cycloalkanes having from 7-16 carbons with oxygen-containing gases in the presence of sparingly water-soluble and sparingly aliphatic or cycloaliphatic hydrocarbon-soluble transition metal catalysts and boron compounds, in particular acids, oxides or esters of boron.
2. Discussion of Background
Compared to the oxidation of cyclohexane with oxygen-containing gases, larger alicyclic hydrocarbons react only slowly to give the corresponding alcohols (OL) and ketones (ON). To accelerate the reaction of cycloalkanes having from 7-16 carbon atoms, boric acid and boric acid derivatives and/or transition metal catalysts can be used in the form of their soluble salts.
Compared with the uncatalyzed reaction, boric acid causes an increase in the reaction rate and conversion rate as described in DRP 552 886, DE 16 43 825 and in the publications by H. Grasemann et at. in Erdxc3x6l, Kohle, Erdgas und Petrochemie 18 (1965), 360 and 22 (1969), 751. The boric acid causes the proportion of ketone in the product mixture to be reduced to a ratio which is typically 1:8 or less (NL 6 505 838).
In order to increase the proportion of ketone in the product mixture, transition metal catalysts can be used. U.S. Pat. No. 4,341,907 describes the use of cobalt 2-ethylhexanoate as catalyst at reaction temperatures of from 130-180xc2x0 C. at a cobalt concentration of at least 1000 ppm. The reaction is conducted in the presence of pyridine. This requires an additional separation stage during workup. Pyridine has a characteristic unpleasant odor. In addition, there are a plurality of literature citations stating that pyridine is suspected of being carcinogenic.
According to DE 11 11 177, at 142xc2x0 C. a conversion rate of approximately 10% and an ON/OL ratio of approximately 1:1 at a selectivity of approximately 50% are achieved. Because of the low selectivity, in addition to the desired ON/OL mixture which results from overoxidation, large amounts of an oily residue are produced which must be discarded or worked-up. The work-up requires a considerable expenditure and only partially leads to dodecanedioic acid as a utilizable product.
DE 22 23 327 describes the use of cobalt naphthenate as catalyst for oxidizing cyclohexane. At 160xc2x0 C., a conversion rate of 6.5% may be achieved in this manner. However, the oxidation of cyclododecane in the absence of cobalt salts is described, using for this boric acid or similar boron compounds as the catalyst.
DE 16 18 077 claims the oxidation of cycloalkanes in the presence of a usual oxidation catalyst and in the presence of from 0.05-3% of alkyl-substituted aromatic hydrocarbons. Specific catalysts include cobalt naphthenate and other soluble cobalt salts. All of the examples of the patent solely are concerned with the oxidation of cyclohexane, the ON/OL mixture being produced at a selectivity of 76-80% and a cyclohexane conversion rate of about 3.5%.
According to GB 14 28 964, a cobalt oxime derivative is used as catalyst for the oxidation of cyclododecane at 165xc2x0 C. and atmospheric pressure. 8.5% of an ON/OL mixture, based on cyclododecane used, is obtained in this manner.
The combination of boric acid and Co/Mn salts as a catalyst is mentioned in Example 5 in U.S. Pat. No. 3,419,615. At a temperature less than 155xc2x0 C., virtually no reaction takes place.
Only at temperatures of from 160-170xc2x0 C. is a conversion rate of about 25% achieved in a batchwise reaction procedure and about 15% in a continuous reaction procedure. Above 170xc2x0 C., unwanted decomposition reactions take place. In this process, from 0.1-10 mol. % of heavy metal salt, based on the total amount, is used as the catalyst.
In the workup of the reaction mixture, the organic phase is usually washed with water or alkali metal hydroxide solutions. The above-mentioned processes use highly water-soluble heavy metal compounds which are transferred to the aqueous phase during work-up. As a result, the process is encumbered with the complex recovery of the heavy metals from the aqueous phase. In addition, the high heavy metal loading in the waste water further restricts industrial usage of these processes.
U.S. Pat. No. 5,767,320 describes the oxidation of cyclohexane in the presence of phthalocyanine complexes of various heavy metals as sole catalyst.
Accordingly, one object of the present invention is to provide a method of catalyzing the oxidation of cycloalkanes to cyclic alcohols and ketones at improved yield and selectivity and particularly improved proportionate amounts of cyclic ketone in the product.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for preparing cyclic alcohols and ketones having from 7-16 carbons, which comprises oxidizing cycloalkanes having from 7-16 carbon atoms in the presence of an oxygen-containing gas and a sparingly water-soluble and sparingly aliphatic or cycloaliphatic hydrocarbon-soluble transition metal catalyst of Group 6 to Group 12 combined with a boron compound, and separating the sparingly soluble transition metal catalyst by mechanical separation after completion of the reaction.
It has now been found that when a sparingly water-soluble and aliphatic or cycloaliphatic hydrocarbon-soluble transition metal catalyst (sparingly soluble complex, complex salt or a salt of a transition metal), in a particular phthalocyanine complex or a substituted phthalocyanine complex of a metal of Group 6 to Group 12 is combined with a boron compound, in particular an acid, oxide or ester of boron, substantial process improvements can be achieved in the oxidation of higher cycloalkanes.
In the process of the invention the starting cycloalkane has a carbon atom content of 7-16, in particular from 8-12, thereby producing the corresponding cyclic alcohol and ketones.
The improvements provided by the present process, in particular, are:
lower reaction temperatures, in particular temperatures below 150xc2x0;
improved conversion rates;
increased proportion of cyclic ketones in the mixture; and
easier work-up of the reaction mixture.
The transition metal catalyst, i.e., a transition metal complex, a transition metal complex salt or a transition metal salt, employed in the present process is sparingly soluble or very sparingly soluble in the organic phase used as solvent, and in particular in the aqueous phase used to work-up the reaction mixture.
The low solubility of the transition metal catalyst of the invention means that the catalyst exists in the reaction mixture virtually completely as a heterogeneous suspension of finely divided solid. With good conversion rates, this produces, as a result, in particular, significant advantages in the work-up and removal of the transition metal, since the metal can be readily separated by mechanical separation processes such as filtration, sedimentation and centrifugation. Because of the very low water solubility of the transition metal complex, transition metal complex salts or transition metal salts of the invention, during the work-up of the reaction mixture, the catalyst only passes in trace amounts into the aqueous phase. The metal loading in the waste water can thus be considerably reduced in comparison to the prior art.
The catalyst isolated from the reaction mixture can be dried and then reused in the synthesis.
Suitable transition metals from which the complexes of the invention as complex salts or salts, in particular, include iron, cobalt, chromium, copper and manganese, and their mixtures. Preferably, cobalt or manganese is employed as the transition metal for the catalyst.
Suitable complexes, in particular, include phthalocyanine and substituted phthalocyanine complexes. The substituted phthalocyanine complexes are usually substituted by from 1-16 substituents which are normally in the 1, 2, 3, 4, 8, 9, 10, 11, 15, 16, 17, 18, 22, 23, 24 and 25 positions of the phthalocyanine skeleton. Preferred substituents include electron-withdrawing groups, and various substituents may be present in the molecule. Non-limiting examples of electron-withdrawing groups include halogen, nitro and cyano groups.
The metal concentration of the transition metal catalyst ranges from 0.0005-5% by weight, preferably from 0.003-1% by weight, based on the total mixture.
Suitable boron compounds include, in particular, the boron acids such as orthoboric acid and metaboric acid, boron oxide and esters of boron such as trimethyl or triethyl borate. Preferably, orthoboric and metaboric acids are used.
The concentration of the boron compound, in particular, boric acid, ranges from 0.1-25% by weight, in particular from 0.1-10% by weight, based on the total mixture.
Preferred solvents include apolar aprotic solvents such as alkanes and cycloalkanes. A particularly advantageous feature of the invention is that the cyclic alkane starting may be used as the solvent.
The oxidation reaction is conducted in the temperature range from 90-160xc2x0 C., in particular from 130-150xc2x0 C., and particularly preferably at from 140-150xc2x0 C.
The reaction is usually conducted at atmospheric pressure. However, a superatmospheric pressure up to about 10 bar, preferably up to 5 bar, can be employed.
The reaction can be conducted batchwise or continuously.
Normally, the oxidative gas employed, in the simplest case, is dried air. However, it is also possible to employ an atmosphere containing a desired oxygen concentration in the inlet gas or exhaust gas in a specific manner by introducing nitrogen, air and/or oxygen into the reactor.
Having now generally described the invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purpose of illustration only and are not intended to be limiting unless otherwise specified.
Examples which relate to the oxidation experiments.
All experiments were conducted in a stainless V4A steel 5 l autoclave. Nitrogen and oxygen were introduced directly into the liquid phase above the agitator. All reactions were conducted isothermally. In the experiments the reaction temperature was controlled by an external heater via the reactor wall and an internal cooling circuit in order to remove the heat of reaction. In addition, a cooling circuit was employed to rapidly cool the products after completion of the reaction. The exhaust gas exited the reactor through a two-stage metal cooler and was freed from entrained products in a downstream gas scrubber.